In recent years, from the viewpoint of environmental protection and fuel consumption rate, a hybrid electric vehicle (HEV), an electric vehicle (EV), and also a fuel cell vehicle are produced and sold, and a new development is in also progress. For those so-called electric powered vehicles, use of a power source capable of charge and discharge is essential. As a power source, a secondary battery such as lithium ion battery or nickel hydrogen battery, an electric double layer capacitor or the like is used. In particular, due to high energy density or high durability against repeated charge and discharge, a non-aqueous electrolyte secondary battery such as lithium ion secondary battery is considered suitable for an electric powered vehicle, and thus various developments are actively in progress.
A non-aqueous electrolyte secondary battery aimed to be used for an electric powered vehicle is required to have high output and high capacity. As a positive electrode active material used for a positive electrode of a non-aqueous electrolyte secondary battery, a solid solution positive electrode active material, which contains a transition metal such as lithium and manganese, is known. In particular, manganese is a relatively abundant resource material that can be easily obtained at low cost, and it has a low environmental burden. Thus, it is desirably used for a positive electrode active material.
The solid solution positive electrode active material is known to elute a transition metal (Ni, Mn) of the positive electrode active material to an electrolyte solution during repeated charge and discharge of a battery. Once the transition metal is eluted, the crystal structure of a positive electrode active material changes, and thus there can be a problem of having lower battery capacity due to insufficient absorption of lithium ions.
To prevent the elution of transition metals, a positive electrode active material having composition formula of xLiMO2.(1-x)Li2M′O3 (in the formula, 0<x<1, M is V, Mn, Fe, Co, or Ni, and M′ is Mn, Ti, Zr, Ru, Re, or Pt) is proposed in the Patent Literature 1.